Implants are used in modern medical technology in a variety of embodiments. For example, they are used to support blood vessels, hollow organs and duct systems (endovascular implants, e.g., stents), for securing and temporary fixation of tissue implants and tissue transplants, but also for orthopedic purposes, e.g., as nails, plates or screws. Frequently only a temporary supporting and/or holding function is necessary or desired until conclusion of the healing process or stabilization of the tissue. To avoid complications resulting from implants remaining permanently in the body, the implants must then be removed by surgery or be made of a material that is gradually degraded in the body, i.e., is biocorrodible. The number of biocorrodible materials based on polymers or alloys has grown steadily. Biocorrodible metal alloys of the elements magnesium, iron and tungsten are known. A stent is a form of an implant that is often used.
Implantation of stents has become established as one of the most effective therapeutic measures for treatment of vascular diseases. The purpose of stents is to assume a supporting function in the hollow organs of a patient. Stents of a traditional design therefore have a filigree supporting structure of metallic struts, which are initially present in a compressed form for insertion into the body and are widened at the site of application. One of the main fields of application of such stents is for permanent or temporary dilatation of vasoconstrictions, in particular constrictions (stenoses) of the coronary vessels, and maintaining their patency. In addition, aneurysm stents, for example, which are used to support damaged vascular walls, are also known.
Stents have a circumferential wall of a sufficient load-bearing capacity to keep the stenosed vessel open to the desired extent and have a tubular base body through which the blood flow passes unimpeded. The circumferential wall is usually formed by a mesh-like supporting structure, which allows the stent to be inserted in a compressed state with a small outside diameter as far as the stenosis in the respective vessel to be treated and to widen it there with the help of a balloon catheter, for example, so that the vessel has the desired enlarged inside diameter. A cardiologist must monitor the positioning and expansion of the stent during the procedure and the subsequent position of the stent in the tissue after the end of the procedure. This may be accomplished by imaging methods, e.g., by X-ray examinations.
The stent has a base body of an implant material. An implant material is a nonviable material, which is used for an application in medicine and which interacts with biological systems. The basic prerequisites for use of a material as an implant material, which is in contact with the biological environment when used as intended, is its biocompatibility. The term biocompatibility is understood to refer to the ability of a material to induce an appropriate tissue response in a specific application. This includes adaptation of the chemical, physical, biological and morphological surface properties of an implant to the recipient tissue with the goal of a clinically desired interaction. The biocompatibility of the implant material also depends on the chronological course of the reaction of the biosystem into which the implant is implanted. Thus, relatively short-term irritation and inflammation may occur, which may lead to tissue changes. Biological systems thus react in different ways, depending on the properties of the implant material. Implant materials can be subdivided into bioactive, bioinert and degradable/resorbable materials according to the response of the biosystem.
Implant materials for stents include polymers, metallic materials and ceramic materials (e.g., as a coating). In the area of biocorrodible stents, the use of magnesium or pure iron and biocorrodible base alloys of the elements magnesium, iron, zinc, molybdenum and tungsten is proposed.
In particular when using biocorrodible materials for implants, e.g., stents, suitable control of the corrosion rate of the respective implant after implantation in the body is a field with a need for further optimization. The corrosion rate is to be implemented so that the implant can also fulfill the proper task or function in the body over the desired period of time. In the case of a stent, the integrity of the stent should be ensured for a period of time which is sufficient to fulfill the respective medical purpose. On the other hand, the advantage of biocorrodible stents is that these stents need not remain in the body for an unlimited period of time or even be removed surgically, but instead are degraded and disposed of by the body after a while. The integrity of a biocorrodible stent should be ensured over a period of time, which is as long as necessary but as short as possible.
In particular, stents having a base body consisting entirely or partially of iron or an iron alloy tend to corrode too slowly. Stents having a base body consisting entirely or partially of magnesium or a magnesium alloy often have a corrosion rate in the body that is too high.